In recent years, there has been a growing demand for accuracy improvement of microfabrication technology such as exposure technology for master optical disks and semiconductor technology. For example, optical disks are fabricated through a master disk production process for producing a stamper from a glass plate and a disk production process for producing optical disks with use of an injection mold with the stamper mounted thereon.
In a typical master disk production process, a master disk production machine emits a visible light, an ultraviolet laser light, or the like from the light source in an ambient atmosphere and focuses the light to have a spot diameter of the level of wavelength by a high power objective lens. With this light, a pit pattern latent image corresponding to information signals or the like to be recorded is formed on a glass plate having a photoresist layer on a principal surface thereof. After cutting the photoresist layer, the glass plate goes through a development process or an electroforming process so as to be a stamper.
While cutting the master disk, the master disk production machine rotates the master disk so as to relatively move the irradiation position of the visible light or the ultraviolet light in the radial direction of the master disk. A problem with such a master disk cutting process using a visible light or an ultraviolet laser light is that the recording resolution of information signals to be recorded is limited due to limitations of a light spot diameter, resulting in preventing high density recording.
Cutting by electron beams is able to form finer patterns compared with cutting by laser beams.
In a typical master disk production machine using an electron beam, however, the electron beam is deflected by magnetic field fluctuations inside and outside of the machine, which leads to a variation of a spot position of the electron beam focused by a focusing unit. This lowers accuracy of the track pitch of master disks or causes reproduction jitter.
The causes of the magnetic field fluctuations inside and outside the machine include variation of earth magnetism due to movement of vehicles, elevators, or the like, electromagnetic waves emitted from power supply systems such as cabinet panels, and magnetism generated by a rotating mechanism or a disk transport mechanism of the master disk production machine.
Especially, the magnetism generated by the rotating mechanism, the disk transport mechanism, or the like causes a significant variation of the electron beam irradiation position. There are therefore proposed various methods for reducing influence of the magnetism generated by the rotating mechanism or the disk transport mechanism, or the like (see Patent Documents 1-3).
Patent Document 1 discloses, as a method of removing noise, a magnetic shield device that blocks magnetic noise attempting to enter a box-shaped magnetic shield room having a wall made of a high permeability member.
The magnetic shield device disclosed in Patent Document 1 includes a detection coil passing through holes of a wall and wound around the wall for outputting an induced voltage corresponding to variation of magnetic noise as a detection signal, a cancel coil receiving an external current so as to generate a magnetic field for compensating for the magnetic noise detected by the detection coil, and a controller for controlling a current to be supplied to the cancel coil based on the output of the detection coil. A pair of Helmholtz coil is used as the cancel coil that generates a magnetic field in the same direction as magnetic flux detected by the detection coil.
Patent Document 2 discloses an information recorder that includes a rotating unit for rotating a master of information recording media, an irradiating unit for irradiating an information recording electron beam onto a recording surface of the master, a transport unit for relatively moving the master and the irradiating unit in a direction parallel to the recording surface, and a vacuum atmosphere forming unit for accommodating the rotating unit and the irradiating unit in a vacuum atmosphere.
According to Patent Document 2, the rotating unit of the information recorder includes a turntable on which the master is placed, a spindle shaft for supporting the turntable, an electromagnetic motor for rotating the spindle shaft, and a magnetic shield unit for shielding against magnetism generated in the electromagnetic motor.
In the information recorder of Patent Document 2, in order to reduce magnetic noise from the electromagnetic motor, the entire electromagnetic motor is surrounded by a magnetic casing (high permeability material) for blocking the magnetism, thereby preventing an electron beam from being influenced by an electromagnetic field generated by the electromagnetic motor.
Patent Document 3 discloses an electron beam irradiation device and a method for eliminating influence of magnetism generated from a rotating mechanism or a master transport mechanism and magnetism in the vicinity of an electron gun. The electron beam irradiation device of Patent Document 3 drives an electron beam deflection electrode according to outputs of plural magnetic detectors disposed in the vicinity of an electron beam generator or in a vacuum chamber, deflects an electron beam in a direction for canceling deflection of the electron beam due to variation of magnetic field, and thus corrects displacement of an electron beam focus spot on a master due to the variation of magnetic field.    <Patent Document 1> Japanese Patent Application Publication No. 2003-124683    <Patent Document 2> Japanese Patent Application Publication No. 6-131706    <Patent Document 3> Japanese Patent Application Publication No. 2002-217086
Referring back to the magnetic shield device disclosed in Patent Document 1, although the cancel coil needs to be highly accurately mounted, it is difficult to accurately mount the Helmholtz coil because of its relatively large size. If the Helmholtz coil is not formed in an ideal shape, the magnetic field generated by the Helmholtz coil is not uniform, resulting in lowering of device accuracy. Moreover, a facility using the magnetic shield device of Patent Document 1 is large and expensive.
Referring to the information recorder of Patent document 2, the electromagnetic motor includes a rotating part, and hence it is difficult to completely shield magnetism of the entire electromagnetic motor by surrounding the entire electromagnetic motor with the magnetic casing (high permeability material).
Another problem with the information recorder disclosed in Patent Document 2 is that, if a large amount of high permeability material is used, the weight of the rotating unit is increased.
Still another problem with the information recorder of Patent Document 2 is that, if a magnetic shield for the entire device is also provided, a magnetic field is generated in the direction of the electron beam irradiation axis in a magnetic shield opening of the irradiating unit and an opening of the magnetic casing of the rotating unit. The generated magnetic field affects the electron beam, which prevents accurate correction of a focal point and results in lowering of exposure quality.
Referring to the electron beam irradiation device of Patent Document 3, according to the method of correcting the displacement of the electron beam focus spot on the master due to the variation of magnetic field and adjusting a focal point of the electron beam relative to a target object, as the plural magnetic detectors estimate a magnetic field in the vicinity of an irradiation point of the electron beam, a disturbance magnetic field of an electron beam injecting part cannot be accurately calculated. Accordingly, the electron beam irradiation device of Patent Document 3 cannot accurately correct the focal point, and therefore exposure quality is lowered.
Further, according to Patent Document 3, for reasons of device configuration, the entire electron beam irradiation device needs to be magnetically shielded for eliminating influence of earth magnetism. However, since there are many constraints on workability and structure such as ventilation and air conditioning facility, this configuration is not so effective for variation of a magnetic field generated inside the shielded device. Furthermore, since the entire device is magnetically shielded, the electron beam irradiation device is large and expensive.